In recent years, the telecommunications industry has witnessed the proliferation of a variety of digital vocoders in order to meet bandwidth demands of different wireline, and wireless communication systems. The name "vocoder" stems from the fact that its applications are specific to the encoding and decoding of voice signals primarily. Vocoders are usually integrated in mobile telephones and in base stations of the communication network. They are also found on computer sound cards and used for Internet telephony. They provide high compression of a digitized voice signal as well as the reverse transformation while maintaining acceptable speech quality.
For the purposes of this specification, the term "vocoder" is defined as a speech coding device that includes two main sections, namely an encoder section and a decoder section. The encoder section receives a speech signal in digitized form, such as PCM (pulse code modulation) samples, calculates speech parameters (compressed form of speech) and transmits those parameters on a communication channel. The decoder section receives the compressed form speech parameters and then synthesizes the speech signal. In a specific example, the synthesis operation produces PCM samples. The purpose of the decoder section, therefore, is to effect a transformation that is the reverse of the encoding operation, namely transforming compressed speech frames into an uncompressed speech signal.
The main advantage of compressing speech is that it uses less of the limited channel bandwidth for transmission. The main disadvantage is a reduction in speech quality.
The rapid growth in the diversity of networks and the number of users of such networks is increasing the number of instances where two vocoders are placed in tandem to serve a single connection. Tandem connections of low bit-rate vocoders are known to cause additional distortions and reduce the quality of the speech signal. One example of such a scenario in a wireless context is a wireless-to-wireless link.
In such a case, a first encoder section is used to compress the speech signal generated by the first wireless user. The compressed speech frames generated by the encoder section are transmitted to a base station serving the local wireless terminal and they are then decompressed (converted to PCM format samples) by the decoder section of the local vocoder. The resulting PCM samples arrive at the remote base station serving the second wireless terminal, over the digital trunk of the telephone network. At the remote base station, the PCM samples are compressed by the encoder section of the local vocoder. The compressed speech signal is then transmitted to the second wireless terminal. A the decoder section at the second wireless terminal decompresses the received compressed speech frames to synthesize the original speech signal from the first wireless terminal.
This method of transmitting the speech signals introduces degradation in the speech quality. This is due to the successive compression/decompression cycles of the signal. A possible solution to this problem is to bypass the decoder section of the first base station and the encoder section of the second base station. With this arrangement, compressed speech frames are directly transmitted from one wireless terminal to the other wireless terminal, rather than being converted to PCM samples and transmitted in PCM form through the PSTN network. However, this solution is only feasible when the two mobile terminals, hence the base stations serving them, employ identical vocoders. If the two terminals involved in a connection utilize different vocoders, it is no longer feasible to bypass the intermediate decompression/compression stages. The "bypass" approach is described in the international application serial number PCT/CA95/00704 dated Dec. 13, 1995. The contents of this disclosure are incorporated herein by reference.
Another possible solution is to equip each wireless terminal with a plurality of encoders and decoders selectively used in dependence of the encoders and decoders provided at the remote wireless terminal. At call set-up time, through a combination of in-band and out-of-band signaling and a negotiation protocol, the most suitable common encoders and decoders are selected by each terminal. Unfortunately, this solution may not always be practical since the provisioning of a number of encoders and decoders implies higher costs. In effect, the complexity, and therefore the cost, of encoders is relatively higher than the cost of decoders. This solution hence requires the use of very powerful digital signal processing devices and consumes a great amount of memory capacity.
Thus, there exists a need in the industry for a device and a method capable of improving the voice quality during connections that may include tandemed vocoders, that can be implemented at a relatively low cost.